Physics Says This Is the Best Way to Deal With Hot Coffee

I like my coffee hot, but not too hot. Is it better to leave the lid on and blow, or just take the lid off?

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Like many people, I enjoy getting a nice cup of coffee. Not that silly sugary stuff like a double-whip, non-fat, vanilla bean, espresso, iced with a twist of lime. No, I get plain old boring black coffee. But there's a problem: It's almost always too hot to drink right away. When life gives you coffee that's too hot, you must find the best way to cool it.

I have two methods to cool off my coffee. Method No. 1 is to remove the lid (coffee usually comes with a lid so you don't spill it). Method No. 2 is to leave the lid on, but occasionally blow over the little hole that you are supposed to drink through.

Which method works the best? Let's find out with a simple experiment. OK, before doing that, I need some things. First, coffee cups. That's easy, just drink coffee and save the cups. Second, I need some temperature values. What is the starting temperature of coffee? It seems 80°C is a common coffee temperature. But what temperature do I like my coffee? Based on my personal preference, I find that 64°C is quite nice.

Now for an experiment. I will pour hot water (the assumption being it is similar to black coffee) into three cups.

Cup 1 has a lid and that's it. This is your standard cup of coffee.

Cup 2 has no lid.

Cup 3 has a lid, but I am going to blow on the opening.

Which method works best? Could I simply sip each cup and make a comparison? Of course, but wouldn't it be better to actually measure the temperature? Yes, that's what I will do. Here are the three cups with temperature probes.

I tried to make Cup 3 as realistic as possible. I didn't blow on it continuously, but rather every once in awhile as though I was busy doing other stuff while waiting for my coffee to cool. And now for the data. Here is plot of the three cup temperatures over the course of a few minutes.

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Right here you can see the answer. Although the starting temperatures were a little different, it's clear that the cup without a lid cooled fastest. Why? Because there are just two ways to cool your coffee. First, you could use heat conduction. By placing the coffee next to something colder, thermal energy is transferred to the colder object, decreasing the coffee's temperature. In my experiment, the only things colder than the coffee are the air and the surface the cup is sitting on. This probably isn't enough to have a significant impact on the coffee's temperature.

The second method is cooling by evaporation. As water on the surface of the coffee evaporates, it leaves behind lower energy water molecules, resulting in a lower coffee temperature. Without a lid, the cup provides a large surface area for evaporation---and thus cooling. If you leave the lid on, you could possibly increase the evaporation rate by blowing over the hole, but this clearly isn't as effective as removing the lid.

Now for one more experiment. Some places a plug in the small hole when you order coffee to go.

Does that help keep the coffee hot? Let's find out. For this test, I repeated the earlier experiment with two cups. Cup 1 has a lid, while Cup 2 has a lid and a plug. Here is the temperature vs. time for the two cups.

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Although it looks like the coffee with the lid-plug is colder, the temperatures are essentially the same. Honestly, I'm surprised. I thought the cup without the plug would be noticeably cooler. I guess the main reason for the plug is to prevent spilling coffee.

Like many people, I enjoy getting a nice cup of coffee. Not that silly sugary stuff like a double-whip, non-fat, vanilla bean, espresso, iced with a twist of lime. No, I get plain old boring black coffee. But there's a problem: It's almost always too hot to drink right away. When life gives you coffee that's too hot, you must find the best way to cool it.